1. Field of the Invention
The present invention relates to a shooting lens for reducing a vibration of an image of a subject and a camera system therefor.
2. Description of the Related Art
There has been a known technique for driving a vibration reduction mechanism to reduce a vibration of an image of a subject due to a hand vibration or the like. Such a known technique includes a vibration reduction mechanism (such as an optical vibration reduction system or the like) and an angular speed sensor. The angular speed sensor detects vibration of a shooting lens and of a camera. The shooting lens decides, from the angular speed, the position of the vibration reduction mechanism to eliminate the vibration of the image (hereinafter referred to as target drive position), and moves the vibration reduction mechanism to the target drive position.
In addition, the shooting lens executes a positional control over the vibration reduction mechanism, moving it back to the center position (hereinafter referred to as center bias control), by feeding back the displacement thereof to the control over the vibration reduction mechanism. The center bias control allows the vibration reduction mechanism to be moved back to the vicinity of the center position thereof. As a result, it is possible to substantially expand the moving range of the vibration reduction mechanism.
Japanese Unexamined Patent Application Publication No. Hei 10-322585 (FIG. 1) (Reference 1) and 10-145662 (FIG. 1 and FIG. 3) (Reference 2) have disclosed an image vibration reduction technique for a video camera. The video camera detects a motion signal from a captured image. Then, the video camera interpolates the motion signal to raise the sampling grade thereof. The video camera improves the vibration reduction performance by feeding back the interpolated motion signal to a target drive position that is updated at high speed.
[Problems of Known Technique]
In the known vibration reduction control technique, a DC offset and a drift contained in an output of an angular speed sensor cause problems, because odd components such as these DC offset and drift have to be removed in order to accurately detect the vibration of a subject image. However, these odd components vary depending on the temperature and use conditions of the angular speed sensor. Thus, the values of the DC offset and drift measured for shipment are not usable for actual shooting. Conventionally, they are separated and extracted from an output of the angular speed sensor when a subject is actually shot.
A vibration of a user's hand has frequency components whose dominant frequencies are in the range from 2 to 7 Hz. On the other hand, the angular speed sensor in a stationary state outputs frequency components whose dominant frequencies are less than 1 Hz. Thus, by the use of the moving average or a low-pass filter, low frequency components are extracted from an output signal of the angular speed sensor, thereby estimating the DC offset and drift in real time.
However, by this known technique, a reference signal has various errors. FIG. 12A, FIG. 12B, and FIG. 12C show a simulation result of a conventional reference signal estimation. In FIG. 12A, the moving average of the angular speed sensor is calculated in accordance with an output signal thereof to obtain a reference signal. The moving average causes a delay in the phase of the drift of the reference signal. In addition, the reference signal contains a vibration component that is not completely smoothened by the moving average. When a reference signal containing an error is removed from the output signal of the angular speed sensor, the angular speed will has an error shown in FIG. 12B.
In FIG. 12C, a thick line represents a result of a variation reduction operation for an angular speed including an error. Although a high frequency component of a hand vibration decreases, the vibration reduction mechanism gradually drifts over time.
As described above, the vibration reduction performance depends on how accurate reference signal of the angular speed sensor can be obtained.
[Problems of References 1 and 2]
In the techniques disclosed in the References 1 and 2, a motion signal is used to reduce a vibration of an image. However, the controlling systems therein are for shooting movies. If these techniques are applied to electronic still cameras, the following problems [1] and [2] will arise.
[1] An electronic still camera acquires a motion signal from an image for monitor display before a shutter release. In this case, the shooting interval of the electronic still camera (for example, 30 frames/second) is several times longer than the shooting interval of a common video camera (for example, 60 fields/second in the NTSC system). In other words, the electronic still camera has a longer sampling interval of a motion signal. Feeding back the motion signal with a long interval to the target drive position cannot achieve sufficient vibration reduction effect.[2] Moreover, in the technique disclosed in References 1 and 2, the motion signal is extrapolated so that the interval of the motion signal matches with the update interval of the target drive position. On the other hand, the electronic still camera uses a motion signal with a long sampling interval. Thus, it is difficult to estimate accurate extrapolation so that discontinuous errors may occur in the extrapolation. The errors in the extrapolation results in errors in the control of the target drive position. As a result, the vibration reduction effect may conspicuously deteriorate.
In the technique disclosed in the References 1 and 2, the motion signal is fed back to the target drive position. On this point, the technique is clearly different from an invention by which the reference signal is corrected with the motion signal. Moreover, in the References 1 and 2, a high-pass filter is disposed in a feedback path for the motion signal. The high-pass filter does not allow low frequency components corresponding to the drift and offset to pass therethrough. Consequently, the technique disclosed in the References 1 and 2 is not able to properly correct the drift and offset of low frequency range.
Moreover, for the electronic still camera, unlike a video camera, photographing with a long exposure (an exposure of 1/15 seconds or longer) needs to be considered. At shooting with a long exposure, the image vibration will arise from a low speed drifting movement. However, in the video camera the low speed drifting movement does not cause the image vibration due to its slow shutter speed.
A very low frequency component of a drift causing the image vibration does not pass through the foregoing high-pass filter. Because of this, the technique disclosed in the References 1 and 2 cannot prevent the image vibration caused by a long-exposure shooting.
[Problem Caused by Synergy Between Motion Signal and Center Bias]
To keep the vibration reduction mechanism at its center position, it may need to increase the feedback gain of the center bias. In this case, strong force returning the vibration reduction mechanism to the center position will occur (hereinafter, this force is referred to as bias power). A strong bias power causes deterioration in the stability of the vibration reduction control; accordingly, it may cause the vibration reduction mechanism to oscillate at worst.
In addition, the inventors of the present invention have found that the feedback of the motion signal to the vibration reduction mechanism causes a problem that the vibration reduction mechanism is likely to oscillate because the stability of the vibration reduction control remarkably deteriorates by a synergistic effect of the feedback of the motion signal and the center bias. The inventors have also found that the feedback of the motion signal to the vibration reduction control causes another problem that the vibration reduction mechanism moves unnecessarily when it stops.
[Problems in Intentional Movement of Camera]
Normally, before shooting a subject, the user needs to change the orientation of the camera to decide a composition of an image to be shot. In addition, the user may need to vibrate the camera to pan a moving subject as it moves.
When the camera is intentionally moved (hereinafter generally referred to as “panning”), since the angle of vibration is larger than the vibration of hand, the output of the angular speed sensor largely varies. When the output varies, it becomes difficult and inaccurate to estimate the reference signal of the angular speed sensor. As a result, the reference signal contains a large error. Accordingly, the vibration reduction would become inaccurate.
In addition, the panning causes the shot image on the camera side to move. As with a residual vibration of a drift output of the angular speed sensor, the motion of the image is detected as a motion signal. When the motion signal is directly fed back to the vibration reduction system, the motion of the panned image would momentarily delay. Alternatively, the motion of the panned image would stop. When the panning is preformed for a long time, the vibration reduction system would exceed its drive limit.